Definition of Terms
- 1st Deriv Mean Motion (/2)
- first derivative of the mean motion divided by two, in units of revolutions per day squared (rev/day²)
- 2nd Deriv Mean Motion (/6)
- second derivative of the mean motion divided by six, in units of revolutions per day cubed (rev/day³)
- Apogee
- point on an orbit farthest from Earth's center of mass
- Apogee Altitude at Epoch
- distance above the Earth's surface of apogee at the elements epoch, assuming a mean equatorial radius of 6378.14 km (3443.92 nmi)
- Argument of Perigee
- angle from the ascending node of an orbit to the perigee, measured in the orbital plane in the direction of motion of the satellite
- Ascending Node
- point where the satellite crosses the equatorial plane headed from south to north; the time at which this occurs is usually used as the elements epoch
- B*
- one-half the product of ballistic coefficient and sea-level atmospheric density, in units of inverse Earth radii
- Ballistic Coefficient
- ratio of the product of drag coefficient (Cd) and reference area (A) to mass (m), giving CdA/m; also defined as the inverse of this quantity when used in other contexts; similarly, object weight (W) is sometimes used in place of object mass with the inverse definition, giving W/CdA
- Eccentricity
- for an elliptical orbit, one-half the ratio of the distance between foci and the semimajor axis; for a circular orbit, eccentricity is zero
- Elements Epoch
- the reference time for the element set; usually the time when the ascending node is reached, in units of calendar year (first two digits), day of the year (next three digits), and day fraction (in decimal format - e.g., 11:30 UTC = (11 hours + 30 minutes/60 minutes/hour)/24 hours/day = 11.5/24 days = 0.47917 days)
- Epoch
- reference time
- Inclination
- angle between the orbital plane and the equatorial plane
- Int'l Designation
- designation assigned by the World Data Center for Rockets and Satellites by international agreement; indicates the year of the launch, the launch number for that year, and the object number for that launch
- Mean Anomaly
- product of the mean motion and the time since perigee
- Mean Motion
- average angular velocity required to complete one revolution on an orbit in the period of the orbit, in units of revolutions per day
- NORAD
- North American Aerospace Defense Command
- NORAD Name
- a non-unique descriptive identifier assigned by NORAD for each Earth-orbiting artificial satellite in their satellite catalog
- NORAD Number
- a unique five-digit number assigned by NORAD for each Earth-orbiting artificial satellite in their satellite catalog; assigned when first observed
- Observed Reentry Time
- time at which eyewitnesses have reported seeing the reentering object, in units of UTC
- Orbit Number at Epoch
- total number of orbits completed at the elements epoch
- Perigee
- the point on an orbit closest to Earth's center of mass
- Perigee Altitude at Epoch
- distance above the Earth's surface of perigee at the elements epoch, assuming a mean equatorial radius of 6378.14 km (3443.92 nmi)
- Period
- time required to make one revolution around an orbit
- Predicted Reentry Time
- time at which a reentering object might first become visible due to aerodynamic heating, corresponding to an altitude of ~120 km (~65 nmi), in units of UTC; for a high-eccentricity orbit, the perigee may actually be much less than 120 km, which may lead to on-orbit fragmentation of the object
- Prediction Epoch
- the most recent elements epoch used to perform the reentry prediction, in units of UTC
- RA of Ascending Node
- right ascension of the ascending node; right ascension is the angular position measured eastward along the celestial equator from the vernal equinox vector to the hour circle of the object
- UTC
- Coordinated Universal Time, the international time standard, corresponding to the time at the prime meridian; informally equivalent to Greenwich Mean Time (GMT) or Zulu time
- Vernal Equinox Vector
- the line pointing from the center of mass of the Earth to the center of mass of the Sun on the spring equinox
